1. Field of the Invention
The invention relates to an extrusion slot die with a flexible lip and a multiplicity of control elements that act on the lip to regulate the slot width so as to extrude flat webs, in particular sheets, of thermoplastics.
2. Description of the Related Art
Extrusion slot dies (100) with flexible lips have, as seen in FIG. 2, a relatively long lip section (102) which, as a result of its small thickness, is elastic and makes possible a change in the slot width by control elements (104) that act on the lip. In the front part (106) of the lip, where the control elements (104) act, the lip must be thickened to support forces from the control elements. In practice, the thickness of the lip in this area is always more than 20 mm, thus considerably limiting its flexibility.
Because the lip is very thick at its tip section in the known extrusion slot dies, the possibilities for precise control of the lip width are limited. For example, due to the melt elasticity, the web leaving the die has a tendency to contract in width while becoming thicker, which can be countered only by reducing the slot width at the sides of the slot. Such fine control is difficult in the prior art.
Also, on every other point of the die local thickness fluctuations of the extruded web can occur due, for example, to inaccuracies in the temperature control that must be compensated for by a countercontrol of the slot width. The rigidity of the front lip area places limits on this precision control.